Water-repellent articles prepared from compositions containing gypsum and hydrophobic agents, such as organopolysiloxanes are described in U.S. Pat. No. 3,455,710 to Nitzsche et al, in which an aqueous emulsion of an organopolysiloxane containing Si-bonded hydrogen is mixed with gypsum and then molded or otherwise formed into articles having water repellent properties.
Organopolysiloxanes containing Si-bonded hydrogen have been mixed with gypsum powder in the absence of water and upon the addition of water have formed water-repellent molded articles or coatings (Chemical Abstracts, Vol. 89, 1978, 29818 w).
It is an object of the present invention to mix organopolysiloxanes containing Si-bonded hydrogen with gypsum to form water-repellent articles. Another object of the present invention is to provide storage-stable gypsum powders containing hydrophobic agents. Still another object of the present invention is to provide gypsum powders which are free of substances which substantially modify the setting behavior of the gypsum. A further object of the present invention is to provide gypsum powders which do not cause "blooming" on articles prepared from the gypsum powders. A still further object of the present invention is to provide gypsum powders in which only water need be added before final shaping of the articles. A still further object of the present invention is to provide gypsum powders which are free of wetting difficulties even when the organopolysiloxane content is greater than 0.5 percent by weight, based on the weight of the gypsum powder.